1. Field of the Invention
The present invention relates to the construction of an anti-earthquake bearing apparatus for use between buildings or structures such as ground facilities or machinery, and foundations or other structures, in a manner capable of avoiding earthquakes.
2. Description of the Related Art
An anti-earthquake bearing apparatus provided with elastic bearing bodies mounted between a structure and its foundation or between the structure and another structure to reduce the response acceleration, and at the same time prevent the resonance of the structure due to an external force such as an earthquake is employed to reduce the energy propagating to the structure such as building and machinery when an earthquake or the like has occurred.
Various elastic bearing bodies have been proposed including, for example those using springs, vibration proof rubber or laminated rubber, and those using various kinds of dampers in combination with the elastic bearing bodies.
A laminated rubber in which elastomer layers such as rubber and reinforcing plates such as metallic plates, are alternated with each other to be integrated in a laminated manner is disclosed, for example, in Japanese Patent Laid-Open Application No. 61-261845 and in Japanese Patent Laid-Open No. 61-14340.
However, if the spring constant in the case of using vibration proof rubber is set to a higher value in order to bear a large load, the expansion of natural period is insufficient causing a lack of earthquake response reduction effect, conversely, if the spring constant is set to a low value, there will be insufficient strength for bearing a structure with a large load.
Further, when using the above described single elastic bearing body with a single value of natural vibration, it is impossible to prevent a resonance for a broad frequency band width of an earthquake.
Furthermore, when using elastic bearing bodies in combination with dampers, the mechanical structure in such composition becomes complicated, whereby time and labor for maintenance are required and higher costs result.
The above described laminated rubber have been proposed for use in anti-earthquake bearing apparatus capable of bearing a heavy weight and increasing the allowable displacement in the horizontal direction.
In an anti-earthquake bearing apparatus using the above described laminated rubber, however, though an excellent response reduction effect is achieved with respect to the horizontal vibration of an earthquake, it is still insufficient in expanding the natural cycle. A technical problem is created such that the response reduction effect in the vertical direction is low. There is also a technical problem that it is difficult to achieve a sufficient response reduction effect for an earthquake vibration having various frequency components because the vibrating system has a relatively short (about 2 to 3 seconds) natural cycle.